I. Introduction
There are numerous industrial applications that need medium voltage and high-power DC-DC conversion ranging from fleet fast electric vehicle charging stations, large data centers, smart distribution systems, and off-shore wind farms. An intriguing technology for handling and implementing such emerging applications is Solid State Transformer (SST). It is fundamentally defined as a structure consisting of at least two power converters and one medium/high frequency (M/HF) transformer in-between. When compared to Low Frequency Transformers (LFTs), SST technology offers many advanced features such as DC connectivity, fault isolation, voltage regulation, and VAR compensation. It can also enable protection and monitoring features that should be supported in future generation grid structures. Studies on SSTs mainly focus on power converter, controls, and M/HF transformer design. Recently, research has concentrated on the various power, frequency, and voltage levels of these transformers. The design studies have been focused on core material, shape, leakage inductance, parasitic capacitance, proximity and skin effect. In addition, examination of MV electric fields, insulation coordination, and partial discharge are of particular importance. Since there are power converters in the SSTs, partial or full soft switching topologies such as Series/Parallel Loaded Resonant (S/PLR), Phase-Shifted Full Bridge (PSFB) and DAB converters are often preferred in these applications to operate at higher frequencies with two or multilevel inverter structures at MV level. The converter structure to be used for excitation of the proposed transformer is shown in Fig. 1.